The Novel, Orally Available Multi-Kinase Inhibitor BAY 73-4506 Targets Myeloma Cells and Their Bone Marrow Microenvironment.

Multitargeted treatment approaches have been shown to be more effective than single agent therapy in multiple myeloma (MM). In addition, agents targeting not only the MM cells directly but also their microenvironment, like bone marrow stromal cells (BMSCs), endothelial cells, and osteoclasts (OCLs) causing enhancement of tumor cell growth, angiogenesis, and MM bone disease, respectively, are promising new treatment modalities for this still non-curable disease.Here we investigated the novel, orally available multi-kinase inhibitor BAY 73-4506, currently in phase I clinical trials, for its therapeutic effect in MM. BAY is a potent inhibitor of angiogenic (VEGFR 1-3, PDGFR-b), as well as oncogenic, kinases (cKIT, RET, FGFR, Raf). We first tested the ability of BAY to suppress MM cell proliferation and survival in a wide array of MM cell lines (MM.1S, RPMI 8226, NCI H929, OPM2, KMS11, KMS 18, INA6, U266, KMS12BM, S6B45), including those resistant to conventional chemotherapeutics (MM.1R, Dox40, LR5). Our data show that BAY is active in all cell lines tested in a low micromolar range equivalent to concentrations achieved in patient plasma during the first clinical trial in solid tumors. Importantly, BAY also overcomes the growth advantage conferred in a BMSC-MM, as well as an endothelial cell-MM, coculture system. BAY treatment abrogates MEK, ERK and AKT phosphorylation in a time and dose dependent manner, followed by induction of apoptosis, evidenced by Annexin staining and DNA fragmentation. Since VEGF signaling pathway is a potent inducer of angiogenesis and BAY targets VEGFR 1-3, we examined anti-angiogenic properties of BAY. This compound inhibits endothelial cell growth and endothelial cell tubuli formation in vitro at concentrations less than 1mM; moreover, BAY markedly inhibits the VEGF-induced cell migration on fibronectin. Activation of MAP kinase is a critical event during OCL differentiation, activation, and survival; BAY inhibits osteoclastogenesis, evidenced by blockade of M-CSF/RANKL-triggered differentiation of mononuclear cells to TRAP-positive osteoclasts, an important marker of osteoclastogenesis. Finally, combination treatment of BAY with dexamethasone shows synergistic effects on MM cell growth and survival. These in vitro experiments on the effects of BAY on MM tumor cells directly, in co-culture with endothelial or BMSCs, as well as on osteoclast differentiation, provides the basis for its evaluation in a murine model of human MM to confirm these promising in vitro effects of this novel multi-kinase inhibitor, finally leading to clinical evaluation to improve patient outcome.